1. Field of the Invention
This invention relates to a radio alarm system for detecting abnormalities such as a fire, and in particular, to a radio alarm system having a central unit and a plurality of terminal devices. Each of the terminal devices is connected to a detector for detecting an abnormal state and is adapted to transmit information in the form of a radio signal based on an abnormality detection signal from the respective associated detector. The central unit is adapted to receive and decode the radio signal transmitted from each terminal device and to give an alarm.
2. Description of the Related Art
Conventionally, a fire alarm system for a building site, etc., has usually consisted of fire detectors arranged at different locations in a building or the like, these fire detectors being connected through wiring to a fire alarm. Apart from this, a radio alarm system has been proposed, according to which terminal devices directly connected to respective fire detectors are arranged on the respective ceilings of those areas requiring supervision. When a fire detection signal is supplied to any one of the terminal devices from the respective associated detector, that terminal device transmits an abnormality detection signal, along with the terminal device address, by radio to a central unit, thereby reporting the abnormal state. In such a radio alarm system, each terminal device is equipped with a battery as the power source, and can be installed at any location where it is required.
In such a radio alarm system, transmission is effected by allocating a number of transmission channels to each of a plurality of terminal devices. The maximum number of terminal devices that allows communication between them and the central unit may, for example, be eight, and the number of channels allocated to each terminal device corresponds to this maximum number of terminal devices that can be provided. If any abnormality is detected by the detector of any one of the terminal devices, that terminal device is first set in a transmitting state, and carrier sensing is performed on one of the channels allocated thereto in order to make a judgment as to whether it is a free channel or not. If it is found that the channel (the first channel in this case) is not being used, it is selected as a free channel. Then, the terminal device is switched over to a transmitting state, and transmits an abnormality detection signal, along with the terminal device address, to the central unit using the channel which has been thus selected through the carrier sensing operation. The transmission of the abnormality detection signal from this terminal device is effected in a continuous manner in which a cycle of a transmission and a rest period are repeated, the respective durations of which may, for example, be eight and two seconds. If the abnormality detection state still prevails at the end of a rest period, carrier sensing is performed again starting from the first channel, i.e., in the order: CH1 (channel 1), . . . , CH8 for the purpose of finding a free channel. After a free channel has been selected through this carrier sensing, the transmitting operation is performed again.
If, in the first carrier sensing, it is found that the first channel is being used by some other terminal device, the object of carrier sensing is instead performed on a second channel. By thus successively performing carrier sensing, carrier sensing is repeated until a free channel is found.
The same number of frequency channels (eight in this case) as are allocated to each terminal device are allocated to the central unit, which is constantly performing carrier sensing successively on the eight channels. When it detects a carrier in any one of the eight channels, the central unit fixes that channel in a receiving state to receive therethrough an abnormality detection signal from one of the terminal devices. It then discriminates and displays the abnormal state, thus giving an alarm.
The generation of an oscillation at the local oscillation frequency to be used in the receiving operation in the carrier sensing effected by each terminal device and the generation of an oscillation at the carrier frequency to be used when performing transmission through a free channel are effected through the setting of dividing ratio data in a PLL circuit.
The PLL circuit is composed of a divider and a phase comparator, and is adapted to compare the division output of a reference oscillator with the division output of a VCO (voltage control oscillator) by means of the phase comparator, performing feedback control of the oscillation frequency of the VCO in such a manner that the output of the phase comparator is reduced to zero. By externally changing the dividing ratio of the divider on the VCO side, a desired oscillation frequency can be obtained from the VCO.
Assuming that the carrier frequency ft1 of channel CH1 is, for example, 429.175 MHz, the local oscillation frequency fr1 for carrier sensing at that time is 407.475 MHz. (It is assumed that the intermediate frequency fi is 21.7 MHz). By setting dividing ratio data in the PLL circuit in such a manner that this local oscillation frequency fr1 is obtained, the carrier sensing on channel CH1 can be effected. If channel CH1 is a free channel, the transmission of an abnormality detection signal can be effected through this channel CH1 by setting, in the PLL circuit, the dividing ratio data on the carrier frequency ft1 of channel CH1.
A problem in such a conventional radio alarm system, however, is that, if two or more terminal devices detect an abnormality simultaneously, the same channel may be selected as a free channel through carrier sensing, that channel being used for the transmission of the respective abnormality detection signals. Such a simultaneous transmission will result in a confusion, and the central unit will regard the data transmitted as ineffective, which means the reception of the abnormality detection signals will become impossible.
Furthermore, in such a conventional radio alarm system, carrier sensing to search for a free channel is also performed starting from the first channel, i.e, in the order: CH1, . . . , CH8, for the re-transmission to be effected after abnormality detection information has once been transmitted from any one of the terminal devices to the central unit. Assuming, for example, that channel CH3 was used in the previous transmission since channels CH1 and CH2 had been found to be in use in the previous carrier sensing, it is quite possible that channels CH1 and CH2 are also found to be in use this time, which means it takes a relatively long time to find a free channel through carrier sensing if a carrier sensing is performed starting from the first channel CH1 in order. This is particularly true of the case where a plurality of terminal devices are operated all at once on the occasion of a test or the like, all the terminal devices transmitting an abnormality detection signal all at once. Since carrier sensing is then started from the first channel in every terminal device, signals collide with each other, with the result that, in certain terminal devices, a lot of time is required for the selection of a free channel. As a result, it takes too much time for the reception and display of the abnormality detection signals to be finally completed in the central unit.
In addition, it may happen, in such a conventional alarm system, that the so called PLL locking cannot be effected even when dividing ratio data is set in the PLL circuit of a terminal device for the purpose of effecting carrier sensing or transmission. The PLL locking is to be effected to allow the PLL loop to operate effectively to realize an oscillation-frequency-control condition corresponding to the dividing ratio. In this case, even a temporary malfunction caused by a noise, etc., may lead to a PLL locking failure, with the result that no transmission can be performed.